Various abbreviations that appear in the ensuing description are defined as follows:
3GPP 3rd Generation Partnership Project
BTS base transceiver station
E-DCH enhanced dedicated channel
E-DPCCH E-DCH dedicated physical control channel
E-DPDCH E-DCH dedicated physical data channel
E-TFC E-DCH transport format combination
E-TFCS E-DCH transport format combination set
FDD frequency division duplex
HARQ hybrid automatic repeat request
HSUPA high speed uplink packet access
IE information element
MAC medium access control
NBAP Node B application part
Node B base station
RNC radio network controller
RRC radio resource control
SF spreading factor
UE user equipment
UMTS universal mobile terrestrial system
UTRA UMTS terrestrial radio access
UTRAN UMTS terrestrial radio access network
WCDMA wideband code division multiple access
The following discussion relates to the 3GPP specification of UTRA, and more specifically to a WCDMA HSUPA (FDD Enhanced Uplink) feature that has been specified in 3GPP release 6, and even more specifically to the computation of uplink gain factors for HSUPA.
As currently specified in 3GPP release 6 (and release 7) the gain factor control is such that that the UE selects a transmit rate by performing E-TFC selection from the E-TFCS. The gain factor for E-DPCCH, and a set of reference gain factors (up to eight) for the E-DPDCH, namely the values, are signaled by the UTRAN. The E-DPDCH gain factors typically vary for each E-TFC. Up to 127 E-TFCs may exist and be used and, thus, for those E-TFCs which do not have reference gain factors signalled by the UTRAN computed gain factors must be produced by the UE and BTS using an equation defined in 3GPP TS 25.214 (Technical Specification Group Radio Access Network; Physical layer procedures (FDD)). Thus, for each E-DPDCH data rate there exists a specific network configured power offset between the E-DPCCH and the E-DPDCH.
The currently defined equation assumes that there is a linear relationship between the data rate and the E-DPDCH to DPCCH gain factor, i.e., if the data rate is doubled then the gain factor is increased so that also the power difference of the E-DPDCH to DPCCH is doubled. This assumption would remain valid if the DPCCH level was independent of the used data rate. However, in practice the DPCCH power level needs to be increased when the data rate increases and decreased when the data rate decreases. The E-DPDCH power level is fixed to the DPCCH power level with the data rate dependent gain factor. Thus, when a data rate changes the gain factor is changed, thereby changing the E-DPDCH power level. However, the data rate change results in a changed DPCCH level (in the same direction), thus amplifying the E-DPDCH power level change.
More specifically the gain factor of an E-DPDCH means the amplitude of the E-DPDCH signal in relation to the DPCCH signal. The power ratio is achieved by squaring the amplitude ratio. The relative power of all the E-DPDCHs over the DPCCH in a multi-code transmission can be achieved by then multiplying the power ratio of a single E-DPDCH by the number of E-DPDCHs in transmission. For example, if a spreading factor 2 is used for an E-DPDCH then it is considered as two spreading factor 4 E-DPDCHs in the gain factor computation, and the amplitude is then corrected by multiplying this with the square root of 2 in the actual transmission.
What is important to note is that the DPCCH level must be sufficient in order to be able to receive the E-DPDCH correctly, as the DPCCH provides the channel estimate required in the reception of the E-DPDCH.
More specifically, the currently defined equation results in the following behavior. A reference E-TFC (jth E-TFC) is used to calculate gain factors for E-TFCs producing higher data rates (E-TFCs j+1 to j+n). This results in E-TFCs j+1 to j+n adequate to obtain a correct power level, if the DPCCH level does not change as a function of data rate. For higher data rates (the E-TFCs j+1 to j+n) the DPCCH level is not sufficiently high to obtain a good channel estimate and, thus, the power control increases the DPCCH level which may result in an unnecessarily high E-DPDCH power level. As a result, cell capacity is wasted. A similar phenomenon can occur for lower data rate E-TFCs, where the DPCCH level is set too high and cell capacity is again wasted.
In short, the current approach is clearly not optimal in terms of cell capacity.
As currently specified, the gain factor of a E-TFC is calculated based on a reference E-TFC (with the gain factor signaled by the network) so that there is a linear relationship of the data rate difference to the power difference. The gain factors affect the signal amplitude, which has a squared impact on the power change since the equation has square roots. Moreover the equation ensures that the power difference over all the E-DPDCHs follows the same relation if multiple parallel E-DPDCHs are being transmitted. The gain factor is set per E-DPDCH, but the equation takes the number of E-DPDCHs into account as well if the reference E-TFC has a different number of E-DPDCHs as the one for which the gain factor is being computed. Multiple parallel E-DPDCHs are used to achieve higher data rates than what a single E-DPDCH can provide.
The current equation and definition thereof can be found in 3GPP TS 25.214, section 5.1.2.5B.2.3, and is as follows:
βed,ref denotes the reference gain factor of the reference E-TFC. Let Le,ref denote the number of E-DPDCHs used for the reference E-TFC and Le,i denote the number of E-DPDCHs used for the i:th E-TFC. If SF2 is used, Le,ref and Le,i are the equivalent number of physical channels assuming SF4. Let Ke,ref denote the transport block size of the reference E-TFC and Ke,i denote the transport block size of the i:th E-TFC, where the mapping between the E-TFCI and the E-DCH transport block size is defined in 3GPP TS 25.321: “MAC protocol specification”.
For the i:th E-TFC, the temporary variable βed,i,harq is then computed as:
                              β                      ed            ,            i            ,            harq                          =                              β                          ed              ,              ref                                ⁢                                                    L                                  e                  ,                  ref                                                            L                                  e                  ,                  i                                                              ⁢                                                                      K                                      e                    ,                    i                                                                    K                                      e                    ,                    ref                                                                        ·                          10                              (                                                      Δ                    ⁢                                                                                  ⁢                    harq                                                        20                    ·                                                  )                                                                        (        1        )            
For the various reasons discussed above, the use of this equation results in a non-optimum usage of cell capacity.